Rechargeable batteries are being used in and designed for varied applications with different requirements for electrical energy. The rechargeable battery systems comprise rechargeable cells which receive electrical energy during charging operations and supply electrical energy to a load during discharging operations. The rechargeable cells may have different chemistries and may include Lithium Ion cells in one example. The numbers of rechargeable cells used in different applications are varied depending upon the requirements of the loads, and the number of cells may be numerous in some implementations.
The rechargeable cells of a battery may be non-uniform, for example, as a result of manufacturing processes of the rechargeable cells. More specifically, different rechargeable cells of a battery may have different or non-uniform parameters (e.g., voltages, internal resistances or impedances, charge-discharge efficiencies). Non-uniformity of cells effectively links the performance of the battery to its least capable rechargeable cell. Furthermore, repeated cycling of rechargeable cells having non-uniform voltage-capacity characteristic curves may lead to increased reduction of capacity of the battery.
In some arrangements, batteries may have hundreds or thousands of rechargeable cells connected together, for example, in series. Typically, the rechargeable cells are charged in series. However, the rechargeable cells may perform differently due to the non-uniformities of the rechargeable cells and the states of charge of the rechargeable cells may vary during charging of the rechargeable cells due to the non-uniformities. More specifically, the voltage-capacity characteristic curves of the rechargeable cells may not be linear during charging or discharging. Some of the rechargeable cells may be fully charged or fully discharged faster than other rechargeable cells due to non-uniformities of the cells which may limit the capacity of the battery which may also be decreased further in subsequent charge/discharge cycles.
FIG. 1 illustrates one conventional approach to account for non-uniformities of the rechargeable cells of a given battery during charging. In particular, a plurality of shunting circuits each including a resistor 14 and logic control switch 16, such as a power MOSFET, may be coupled in parallel with respective rechargeable cells 12. The shunting circuits operate to shunt excess current around respective ones of the rechargeable cells 12 which have reached their maximum voltages quickly, thereby partially or fully bypassing such rechargeable cells 12 and slowing the charging of such cells 12.
The amount of current which may be shunted using the shunting circuits may be limited due to heat generated by resistors 14 conducting the excess current, especially in applications using enclosed battery packs. Accordingly, charge currents may be limited in some implementations to avoid generation of excessive heat. Furthermore, these circuits may not be effective in relatively large batteries having a relatively large number of cells if capacities of the rechargeable cells differ by more than a few percent.
At least some aspects of the disclosure are directed towards improved charging devices, electrical systems, and associated methods as described in more detail below.